The present invention relates to an apparatus for controlling a power converter, and in particular to an apparatus for controlling a power converter suitable for improving the output wave-form while satisfying the control requirement for the minimum pulse width of the semiconductor device of the main circuit.
As conventional apparatuses for controlling a power converter such as inverter or converter, apparatuses using the PWM (Pulse Width Modulation) control scheme are widely known. In the pulse width modulation scheme, semiconductor devices of the main circuit are turned on and off to cause commutation a number of times, and the pulse widths of a plurality of square-wave pulses are modulated and controlled.
As an example, a current type inverter system including a combination of six semiconductor devices such as GTOs having the reverse-blocking function or series circuits of diodes, GTOs and transistors, DC reactors and capacitors for each of the three-phase circuit has been proposed as described in proceedings of National Conference of the Institute of Electrical Engineers of Japan, 1985, No. 502, page 587. This circuit has configuration as shown in FIG. 1. A quiet system has thus been realized by using the simple configuration. Numeral 1 denotes three-phase AC power supply, numerals 2 and 6 overvoltage suppressing capacitors, numeral 3 a current type converter having 6 GTOs, numeral 4 a DC reactor, numeral 5 a current type inverter having 6 GTOs, and numeral 7 denotes a load.
This system is originally configured to produce sinusoidal output voltage. Even when a general-purpose motor is connected as the load 7, therefore, only extremely small noises are generated. So long as a special contrivance is not devised, a square-wave current is originally outputted, resulting in a problem of torque ripple.
As for this problem of output current, a scheme in which a pulse pattern is so generated as to produce a sinusoidal output has been proposed in JP-A-60-98876. This results in a significant effect.
Even in this known example, however, there is a fact which is considered to pose a problem. In case a very low frequency current output is needed, the problem is caused. In order to realize an equivalent output current I of the inverter, as shown in FIG. 2A current commands i.sub.0 having width controlled in accordance with PWM (pulse width modulation), as shown in FIG. 2B which shows the area in vicinity of 0-cross point of FIG. 2A in an enlarged scale, are outputted to a phase. When a current I has a value near zero, the pulse current command i.sub.0 must be very narrow in pulse width. Especially when the output frequency of the inverter is very low, narrow width pulses must be outputted in succession in order to obtain a sinusoidal output.
However, GTOs and transistors used in the power converters 3 and 5 are usually subject to limitation of minimum pulse width beyond which the pulse width should not be narrowed because of the device rating as described in "New Drive Electronics", pages 278-279, published by Denki Shoin in Japan, for example. From the viewpoint of preventing the device breakdown, the limit value in pulse width is approximately 30 to 100 .mu.sec.
When a low frequency is outputted, therefore, the width of the pulse current i is limited to the above described limit value and remain constant as represented by i.sub.1 although the current I should vary. This results in a problem that the sinusoidal wave of the output current is not attained in a part thereof. This phenomenon occurs in the vicinity of zero crosspoints for each of three-phase outputs and affects outputs of other phases. At intervals of 60.degree. in electrical angle, therefore, torque shocks are generated in the vicinity thereof.
The problem has heretofore been described by taking the output of the inverter as an example. A similar phenomenon is true of the converter. In this case, the input current waveform of the converter falls off the sine wave at intervals of 60.degree. in electrical angle. Higher harmonic currents are thus generated for the power supply.
In the above described prior art, the effect of components of the power converter on the output having the minimum pulse width was not considered. And there were problems in the input and output waveforms.
The present inventors proposed novel apparatuses for controlling power converters in U.S. application Ser. No. 2283, "METHOD AND APPARATUS FOR CONTROLLING POWER CONVERTER", H. Inaba et al., filed on Jan. 1, 1987 (Priority claimed application: Japanese Patent Application No. 2875/86) and U.S. application Ser. No. 2199, "AN APPARATUS FOR CONTROLLING POWER TRANSDUCERS OF THE PULSE WIDTH MODULATION (PWM) CONTROL TYPE", H. Inaba et al., filed on Jan. 1, 1987 (Priority claimed application: Japanese Patent Application Nos. 2876/86 and 2877/86). However, the effect of components of the power converter on the output of the minimum pulse width has not been mentioned in the above described U.S. Applications.
An object of the present invention is to provide an apparatus for controlling power converter capable of making the input and output waveforms sinusoidal up to the ultra low frequency region.